Touch panels can be classified according to the position detection method into an optical type, an ultrasonic type, a capacitive type, a resistive film type, and the like. In particular, the resistive film type has a relatively simple structure and thus is cost-effective so that it has come into wide use in recent years. For example, resistive film type touch panels are used for automatic teller machines (ATMs) in banks and for display panels of transportation ticket machines and the like.
The resistive film type touch panels are configured to include a pair of transparent conductive films arranged opposite to each other with a spacer interposed therebetween, in which an electric current is allowed to flow through the upper transparent conductive film, while the voltage at the lower transparent conductive film is measured. When the upper transparent conductive film is brought into contact with the lower transparent conductive film by pressing with a finger, a pen or the like, the electric current flows through the contact portion so that the position of the contact portion is detected.
Conventionally, the so-called conductive glass is well known as such a transparent conductive film, which comprises a glass and an indium oxide thin film formed thereon. Since the conductive glass has a glass substrate, however, it has low flexibility or workability, and cannot be used for certain purposes in some cases.
In recent years, therefore, transparent conductive films using various types of plastic films such as polyethylene terephthalate films as their substrate have been used, because of their advantages such as good impact resistance and light weight as well as flexibility and workability. However, such conventional transparent conductive films not only have the problem of low transparency due to high light reflectance of the thin film surface but also have low scratch resistance or low bending resistance with respect to the conductive thin film so that they have problems in which they can get scratched to have an increased electrical resistance or suffer from disconnection during use. The conventional transparent conductive films also have low environmental resistance and thus have a problem in which the surface resistance can easily change in a high-temperature, high-humidity atmosphere so that the reliability can be poor at high temperature and high humidity. In recent years, the market for touch panels to be installed in outdoor smartphones, car navigation systems and the like is expanding, and therefore, there is a strong demand for improvements in the high-temperature, high-humidity reliability of touch panels.
To solve these problems, attempts have been made to improve transparency, durability and the like with a two-layer structure of transparent conductive thin film formed on a film substrate. For example, it is proposed that a first transparent conductive thin film with a small crystal particle size is formed on a film substrate, and a second transparent conductive thin film with a large crystal particle size is formed thereon, so that transparency, pressure resistance, durability, and the like can be improved and curling properties and the like can be reduced (Patent Literature 1: JP-A No. 2003-263925). It is also proposed that first and second transparent conductive thin films which differ in oxygen content and nitrogen content are formed on a film substrate so that pen input durability can be improved (Patent Literature 2: JP-A No. 2003-151358). However, the techniques disclosed in these literatures cannot achieve satisfactory reliability at high temperature and high humidity.
It is also proposed that a two-layer structure of transparent conductive thin film is formed which comprises an indium-tin complex oxide thin film with a low SnO2 content (3 to 8% by weight) provided on a film substrate and another indium-tin complex oxide thin film with a high SnO2 content (10 to 30% by weight) provided thereon, so that transparency can be improved and that a rise in surface resistance can be suppressed in an annealing step for processing a touch panel or in a drying step for printing silver electrodes or spacers (Patent Literature 3: JP-A No. 10-49306). However, a transparent touch panel electrode composed of the transparent conductive thin films disclosed in Patent Literature 3 does not have sufficient mechanical strength and thus cannot achieve satisfactory pen input durability.
By the way, in recent years, the market for touch panels to be installed in smartphones, personal digital assistances (PDAs), game computers, and the like is expanding, and the frame part of touch panels becomes narrower. This increases the opportunity to push touch panels with fingers so that not only requirements for pen input durability but also requirements for surface pressure durability is satisfied. However, the techniques disclosed in the patent literatures cannot achieve satisfactory pen input durability and thus can never achieve satisfactory surface pressure durability.